Young people with special educational needs and disabilities (SENDs) and their families have been particularly hard hit by the COVID-19 pandemic. In this longitudinal study, sibling conflict in these families during and after the first lockdown in the United Kingdom was investigated. Online questionnaires were completed by 504 parents of young people with SENDs at four time points between 23 March 2020 and 10 October 2020 (over half completed the questionnaire at multiple time points). As lockdown progressed, young people with SENDs were more likely to be picked on or hurt by their siblings compared with earlier stages of the lockdown but there was no change in how frequently they harmed or picked on their siblings. After lockdown, both perpetration and victimization decreased but not to the same rates as the first month of lockdown. Young people with SENDs with severe or complex needs were somewhat protected from sibling conflict. Findings are discussed with reference to implications for support and planning for future pandemics.